We have recently developed reagents and methodology to detect individual antigenic variants of Trypanosoma rhodesiense in mixed variant populations. This recent human isolate develops a chronic, long-term fluctuating parasitemia in mice. This model system will enable us to study the characteristics of T. rhodesiense infection which closely mimics the disease in man. The overall aim of the project is to study the relationship between trypanosomes with their capacity to vary antigenically, the host response to the organisms, and the concomittant immunosuppression which occurs during chronic infection. The specific aims of the project are: (1) to study the course of antigenic variation in a chronic model of infection with relapsing parasitemia and to define the factors which affect that variation; and (2) to study cellular and humoral immune responses, including immunosuppression to trypanosome antigens during a chronic relapsing infection.